Double sized icons in UI?
I am still figuring out how to selectively edit icon size, but the line and file you are looking for is #define ICON_DEFAULT_HEIGHT 16 and #define ICON_DEFAULT_WIDTH_16 in UI_interface_icons.h. For example, changing this height and width to 32 instead of 16 will double all of your icons; it would be ideal if you're scaling the rest of your interface (e.g. button padding) proportionately. This works for Blender source version 2.79. Hope this helps!